1. Field of the Invention
This invention relates to tension indicating devices and more particularly relates to devices for visually indicating the tension in mine roof bolts which are also known as mine wall bolts, rock bolts and the like.
2. Description of the Prior Art
The art of roof bolting in mining tunneling and so on is well known and widely practiced. It is equally well known that the efficacy of the system depends upon the maintenance of the tension in the bolts in a predetermined range, that is at less than a predetermined maximum and at greater than a predetermined minimum acceptable tension.
At installation, the use of a torque wrench is some assurance that the bolt is initially in proper tension condition, although even this is not sure because of various imponderables such as variations in the screwthreading and in friction losses. However, even if the initial tension is correct, it will not necessarily remain so. It is noted that the United States Department of the Interior, Mine Enforcement and Safety Administration, indicates that the prime cause of accidental deaths in mines is the failure of mine roofs and/or ribs.
It is important from the point of view of safety and the general well-being of the mine that variations in tension outside of a permitted range be detected and corrected. This involves periodic inspection of the bolts, which are usually not easily accessible and often all but invisible. These and other difficulties have detracted from the value of known indicating devices.
The prior art includes several devices for indicating the tension in mine roof bolting. These devices may be seen by reference to U.S. Pat. Nos. 2,464,152; 3,104,645; 3,169,440 and 3,329,058. These devices are not totally satisfactory and have not found univeral or wide use because they were not passive and easily seen from a distance, they indicated over or under tension only, they were complicated and expensive to produce and/or install and/or they required electronic apparatus. More recent prior art devices have utilized chemical substances which change color as the tension in the bolt changed. These devices have also been found to be less than totally satisfactory as they are relatively expensive and/or difficult to see.
It is noted that at the present time, the most common mine bolt tension testing and monitoring system comprises a highly paid employee who manually tests mine bolts with a torque wrench.